An optical modulator is a key component for realizing high speed optical communication systems. An optical modulator is configured by using for example a LiNbO3 substrate. An optical modulator configured by using a LiNbO3 substrate is sometimes referred to as an LN modulator. LN modulators can achieve high speed modulation and low chirp, and thus have been put into practical use for optical communication systems of 10 Gbps through 40 Gbps. Also, 100 Gbps-LN modulators that generate a polarization multiplexed optical signal have also been put into practical use in order to realize data communications at higher speeds.
FIG. 1 illustrates an example of an optical modulator. The optical modulator illustrated in FIG. 1 includes a pair of Mach-Zehnder modulators 101X and 101Y in order to generate a polarization multiplexed optical signal. The Mach-Zehnder modulator 101X includes a pair of Mach-Zehnder modulators 101XI and 101XQ, while the Mach-Zehnder modulator 101Y includes a pair of Mach-Zehnder modulators 101YI and 101YQ. Each of the Mach-Zehnder modulators 101XI, 101XQ, 101YI and 101YQ includes a pair of parallel optical waveguides formed on the surface area of a substrate 100. Also, in each of the Mach-Zehnder modulators 101XI, 101XQ, 101YI and 101YQ, a signal electrode 102 is formed near one of the pair of the optical waveguides.
An input optical waveguide 103 is formed so that it branches input continuous wave light and guides the branched portions to the Mach-Zehnder modulators 101X and 101Y. The continuous wave light input to the Mach-Zehnder modulator 101X is guided to the Mach-Zehnder modulators 101XI and 101XQ, and the continuous wave light input to the Mach-Zehnder modulator 101Y is guided to the Mach-Zehnder modulators 101YI and 101YQ.
The continuous wave light input to the Mach-Zehnder modulator 101XI propagates through a pair of optical waveguides. In this situation, when a data signal is fed to the signal electrode 102 of the Mach-Zehnder modulator 101XI, the refraction index of the optical waveguide near that signal electrode 102 changes. Therefore, an optical signal output form the Mach-Zehnder modulator 101XI represents a data signal fed to the signal electrode 102. In other words, the Mach-Zehnder modulator 101XI can generate a modulated optical signal that represents a data signal. Similarly, each of the Mach-Zehnder modulators 101XQ, 101YI and 101YQ generates a modulated optical signal that represents its corresponding data signal.
Optical signals output from the Mach-Zehnder modulators 101XI and 101XQ are combined so as to be guided to an output optical waveguide 104X, while optical signals output from the Mach-Zehnder modulators 101YI and 101YQ are combined so as to be guided to an output optical waveguide 104Y. Note that phase difference π/2 may be given between the Mach-Zehnder modulators 101XI and 101XQ, and phase difference π/2 may be given between the Mach-Zehnder modulators 101YI and 101YQ.
Modulated optical signals output from the optical waveguides 104X and 104Y are combined by a polarization beam combiner 110. In other words, a polarization multiplexed optical signal is generated. The polarization beam combiner 110 may be part of the optical modulator.
Note that an optical device having a plurality of optical modulators is disclosed by for example Japanese Laid-open Patent Publication No. 2010-286770. Also, an optical modulator in which a plurality of optical modulation portions are arranged in parallel is disclosed by for example Japanese Laid-open Patent Publication No. 2010-185977.
It is sometimes demanded that the size of an optical modulator be reduced in order to make the optical transmission device compact. For example, in order to reduce the length of the substrate 100 in the propagation direction of optical signals, the signal electrodes 102 are formed in such a manner that spacing is smaller between the signal electrodes 102 in the propagation direction of optical signals as illustrated in FIG. 2. Spacing S illustrated in FIG. 2 is for example about 1 mm.
As illustrated in FIG. 3, data signals are fed to the optical modulator via a connector 120. In this example, the connector 120 is provided with terminals T1, T2, T3 and T4 to which data signals corresponding to the Mach-Zehnder modulators 101XI, 101XQ, 101YI and 101YQ are input. However, it is difficult to make the spacing (S1, S2 and S3) narrower sufficiently between the terminals of the connector 120. Note that spacing S1, S2 and S3 are 3.6 mm, 10.8 mm and 3.6 mm, respectively according to the OIF (Optical Internetworking Forum). Because of this, a relay board 130 is provided between the substrate 100 and the connector 120. On the relay board 130, wiring patterns 131 are formed so that the pitches between the terminals of the connector 120 are converted into the pitches between the signal electrodes 102 at an edge of the substrate 100.
In addition, in order to increase the quality of a polarization multiplexed optical signal generated by the optical modulator, it is preferable that skews between data signals fed to the Mach-Zehnder modulators 101XI, 101XQ, 101YI and 101YQ be adjusted to be small sufficiently. For this purpose, the relay board 130 is designed so that the respective conductive patterns (wiring patterns 131, signal electrodes 102 and electrodes between P and R (such as the electrode between P1 and R1)) have roughly the same length (propagation time), the respective conductive patterns extending from terminals T1, T2, T3 and T4 of the connector 120 to R1, R2, R3 and R4 via the modulation starting points (P1, P2, P3 and P4) of the Mach-Zehnder modulators 101XI, 101XQ, 101YI and 101YQ. In this example, modulation starting points P (P1-P4) represent the positions at which the conductive patterns arriving at the Mach-Zehnder modulators from the connector 120 via the relay board 130 first arrive at the Mach-Zehnder modulators. For example, P1 in FIG. 3 represents the modulation starting point of the Mach-Zehnder modulator 102XI, and P4 represents the modulation starting point of the Mach-Zehnder modulator 102YQ. Also, R1-R4 represent the points at which beams of light input to the optical modulator chip arrive at the respective Mach-Zehnder modulators at the same time. In such a case, the relay board 130 is designed in such a manner that the arrive time of an electric signal arriving at R1 from terminal T1 via the wiring pattern 131, the signal electrode 102 and modulation starting point P1 is roughly the same as the arrival time of an electric signal arriving at R4 from terminal T4 via the wiring pattern 131, the signal electrode 102 and modulation starting point P4.
However, making the arrival times of electric signals arriving at points R (R1-R4) of the Mach-Zehnder modulators 101XI, 101XQ, 101YI and 101YQ from the connector 120 roughly the same results in longer patterns for the wiring patterns 131 formed on the relay board 130. This also results in greater width W of the relay board 130. For example, the lengths from the connector 120 to the modulation starting points of the Mach-Zehnder modulators are in a range of approximately 5 mm through 8 mm. Also, width W of the relay board 130 is about 3 mm through 4 mm. Thus data signals may be attenuated on the routes from the connector 120 to the Mach-Zehnder modulators 101XI, 101XQ, 101YI and 101YQ. In particular, when a data signal has a high rate, that data signal may attenuate greatly.